172 Mr. J. G. Jeffreys on the Marine Testacea 



and varieties, and a knowledge of the conditions which influence 

 the transition of one to the other, before any satisfactory theory 

 can be established. Whether the term " species" used by na- 

 turalists to denote distinct assemblages of animals and plants 

 which have certain characters in common, has indeed any 

 foundation in nature, is another question ; but it is extremely 

 difficult, if not impossible, to say, with any degree of certainty, 

 what modification of form (to an extent which we should now 

 consider amounts to specific distinction) may have taken place 

 in any race of animals, and particularly in the Invertebrata, 

 during the lapse of so many thousands or myriads of years as 

 have been assigned to the tertiary period. We know the great 

 change which is continually occurring in the form of recent 

 species caused by a difference of station or habitat, food, and 

 many other conditions, and to which we give the name of 

 " variety." How far then is it right to pronounce, without 

 doubt or hesitation, that any of the tertiary species differ from 

 living analogues ; taking into account not only the gradual mo- 

 dification of form which I have before adverted to as probable, 

 but also the absence of numerous links both in fossil and recent 

 species ? I am however satisfied that the proposed distribution 

 of the European MoUusca into any definite provinces or regions 

 is not warranted by a mature consideration of those geological 

 and conchological data which we at present possess. 



It is by no means certain that any definite provinces or re- 

 gions originally had any existence. We know that some species 

 of shell-fish are what is termed " cosmopolite," and are found 

 in every part of the world at various depths of the sea. The 

 most familiar instance that occurs to me is Suxicava rugosa, 

 which, according to Sir Charles Lyell (Principles of Geology, 

 p. 650), " is spread over all the North Polar seas, and ranges 

 in one direction through Europe to Senegal, occurring on both 

 sides of the Atlantic ; while in another it finds its way into the 

 North Pacific, and thence to the Indian Ocean. Nor do its 

 migrations cease till it reaches the Australian seas." This 

 species of shell-fish is found in the sublittoral and laminarian 

 zones (between low-water mark and fifteen fathoms, or there- 

 abouts), but never in deep water, on the coast, perforating sub- 

 marine limestone rocks. Now, although the fry of the Saxicava 

 is undoubtedly free during the short period of its first stage, 

 before the shell is developed and its boring powers brought into 

 action, and it can therefore be wafted some distance, I am not 

 aware that any marine current or stream sets from one side of 

 the Atlantic to the other, nor that there exists any intermediate 

 station, fitted for the reception and habitat of the animal, where 

 it could settle and propagate a succession of emigrants to con- 



